If outright sportiness is what u are after, then the F10 M5 is definitely a wrong choice even though i won't say your 550 test drive was a good indicator. The F10 M5 doesn't feel heavy under most driving condition, but its size becomes apparent in tight places. I test drove the 991 C2S and din like it enough over my E92 M3. The sound and interior ergonomics was to me, not up to scratch compared to my M3. I was very disappointed with the C2S as i had high hopes for it before the drive. Having owned the current M3, it gets that much tougher to find a replacement for it. M really have their work cut out for them in the next M3.

The 550 drive was really just to checkout the overall dimensions of the car and the view from the drivers seat. The drive in the M6 though can't be that much different from the M5.

It is a hard decision to leave the E90, but after 4 years I am getting to the point where I am at least considering it. I was actually the other way, thinking that I wouldn't like the 991 that much (Ive spent time in both 997.1 and .2 and never seriously considered them), but there was quite a lot i did like and not much i didn't. Right now I'm looking at an October allocation, so I have plenty of time still to spend more time with the 991 before making a decision.